For two-way transmission of messages between two stations equipped with such antennas it is convenient to use mutually orthogonal planes of polarization for incoming and outgoing waves lying within a common frequency band. An essential requirement in such a case is the minimization of cross-coupling between the two types of polarization; this calls for an effective suppression of side lobes in the radiation pattern. A high efficiency in both transmission and reception is, of course, also required.
In so-called front-fed antennas, an important design parameter from the viewpoint of efficiency is the ratio R=f/D where f is the focal distance and D is the diameter of the paraboloidal reflector surface. Conventional means for the suppression of side lobes include the provision of an absorbent collar peripherally surrounding the reflector to reduce so-called spillover.
The cross-polarization field E.sub.x is generally given by the expression ##EQU1## where .rho. is the radial variable used for integrating the field over the reflector surface, .THETA..sub.p and .PHI..sub.p are angular coordinates determining the field of radiation, and .alpha. is the relative aperture of the feed given by r/.lambda..sub.o, r being the radius of the feed aperture confronting the reflector and .lambda..sub.o being the median wavelength at the center of the band of operating frequencies. Function g will be described in greater detail hereinafter.
Thus, effective suppression of cross-coupling requires substantial elimination of the field E.sub.x.
In a paper entitled "Feed Design and Method for Reflector Antennas", presented by us at the European Microwave Conference held in Brussels in September 1973, we have discussed the relationship of the efficiency of a front-fed parabolic antenna and the aforementioned parametric ratio R=f/D. We have since determined, however, that the conditions for maximum antenna efficiency .eta. do not yield the best results for the elimination of cross-coupling between waves polarized in mutually orthogonal (e.g. horizontal and vertical) planes.